In the art of weaving machines many devices are known by which the shed formation is carried out, i.e. the heald frames for looms are cyclically moved to the desired positions to provide a certain predetermined pattern of the fabric being worked out.
In all these devices, mechanical reciprocating members are provided which, operated by a rotating drive shaft, cause the heald frames to move between two working positions, namely top and bottom positions, by laws that in all the presently existing devices are strictly cyclically established for each frame. Thus, it occurs that when the fabric pattern has to be changed, said mechanical members must be operated on by adjustments, registrations, replacement of parts, with serious disadvantages as to the continuity in production and capability thereof.
The foregoing is apparent in view of some of the most wide spread designs of shed forming devices, such as the one comprising a pack of cams which by means of oscillating feeling roller levers transmit suitable movements to the heald frames. In such a device, when it is desired to change the sequence of movements for the heald frames, the pack of cams has to be each time changed, which is a rather substantial expense for the weaver, who is obliged to store a large supply of different cams to provide for all the desired dobbies.
Similar drawbacks also occur with another type of shed forming device, that is the eccentric type of device, the use of which is particularly suitable for taffeta weaving. Instead of the conventional knives, this device has circular eccentrics having rocking connecting rods mounted thereon, so as to convert the rotary motion of the shaft into a reciprocating motion of the heald frames through suitable linkages. Also these devices are unsuited (and accordingly have not been hitherto adopted) for shed formation in fabrics of any pattern owing to the drawback that in each cycle the movement of each heald frame between either of two alternate positions is to be always provided. On the other hand, up to now devices were not known for causing a smooth and fast release or restraint according to predetermined requirements of the individual eccentrics to the motion of the drive shaft, so as to vary the position of the heald frames, as desired.
This occurs also in the case of a third type of shed forming devices, that is devices comprising a polycentrical cam and a pair of opposing feeling rollers guided on straight slides, one of which controls the heald frame.